1. Field of the Invention
The present invention relates to a gas discharge tube used as an ultraviolet light source for a spectrophotometer, liquid chromatography, or the like.
2. Related Background Art
A gas discharge tube is a discharge light source using a positive column light emission by arc discharge of a gas filled in a tube. As a typical gas discharge tube, a deuterium discharge tube in which ultraviolet light is emitted by discharge of filled deuterium is well known. This deuterium discharge tube is mainly used as an ultraviolet continuous spectrum source used for a spectrophotometer or the like. In this discharge tube, very small variations, i.e., variations of 0.01% or 0.001%, in output pose a problem during long-time continuous lighting. For this reason, strict characteristics are required in many cases.
In a conventional side-on type deuterium discharge tube which extracts light from the side portion of the tube, a glass envelope incorporates a light-emitting portion for extracting light in accordance with arc discharge. Deuterium gas is filled in the envelope at about several Torr. The light-emitting portion is constituted in a metal discharge shielding box, mounted on a stem, and connected to an external power supply through a lead line.
In the light-emitting portion, a thermionic cathode for emitting thermoelectrons, an anode for receiving the thermoelectrons, and a focusing electrode for focusing arc discharge which occurs between the thermionic cathode and the anode are accommodated in the metal discharge shielding box in a state (floating state) wherein they are not in contact with constituent elements except for the lead line.
The operation will be described below. A power of about 10 W is applied to the thermionic cathode for 10 to 60 seconds before discharge to preheat the thermionic cathode. When the thermionic cathode is sufficiently heated and ready for arc discharge, a trigger voltage of 350 to 500 V is applied between the anode and the thermionic cathode, thereby starting arc discharge. At this time, the path of thermoelectrons is limited to only one because of convergence by the focusing electrode and the shielding effect of the discharge shielding box. More specifically, the thermoelectrons emitted from the thermionic cathode pass along the path converged by the focusing electrode and are received by the anode. An arc ball is generated by arc discharge in a space in front of the focusing electrode on the opposite side to the anode. Light extracted from positive column light emission caused by this arc discharge is projected toward the front side of the anode.
Not to interrupt this optical path, the thermionic cathode is arranged in the discharge shielding box at the side portion along the light projecting direction. After discharge is started, the entire deuterium discharge tube generates heat due to the arc discharge, and the thermionic cathode also receives this heat. Therefore, to prevent overheat of the thermionic cathode, the power applied to the thermionic cathode after discharge is decreased to 1 to 2 W. The heat value due to discharge is very large, so there is a water-cooled type deuterium discharge tube which cools the entire discharge tube by cooling water.
Independent of this prior art, a gas discharge tube having a ceramic discharge vessel commonly used as an envelope is known. In this deuterium discharge tube, ultraviolet light is extracted from an anode side. A thermionic cathode, an anode, and a focusing electrode are accommodated in a ceramic discharge shielding box in a state (floating state) wherein they are not in contact with constituent elements except for a lead line. Such a deuterium discharge tube is described in detail in, e.g., Japanese Patent Laid-Open No. 4-255662.